A safety device of the type discussed is recommended for all types of apparatus used by an operator, in order to protect the operator against accidents or to avoid unsupervised operation of the apparatus used by the operator. A safety device of this type is recommended, for example, for a grinder, milling machine or the like, as well as for a water or foam type fire extinguisher of a fire fighting device, an acetylene torch, a spray head for paint work or metal die casting, etc. Such a safety device is of particular importance for a sandblasting device of a sandblasting unit for pressure sandblasting--dry blasting, moist blasting, wet blasting--where such a safety device acting, so to speak, as a deadman's switch is required by safety regulations. Without intending to limit of the filed of application of a safety device of the type discussed, this safety device is primarily explained below by means of the example of a sandblasting device.
In a known safety device for an apparatus to be manipulated by an operator (DE-U 86 13 771), particularly intended and usable for a sandblasting device of a sandblasting unit, where the device to be manipulated by the operator is connected via a supply line for a supply means, in this case compressed air, to a supply source, the operation of the device is controlled from the supply source. A control device is provided on the supply means source, by means of which the input of the supply means can be controlled. The control device is actuated by an operating sensor disposed on a handgrip of the device. The operating sensor is connected via a cable to the control device. When the actuation of the operating sensor by the operator ceases, the device is switched off.
The operating sensor of the known safety device is designed as an electronic proximity sensor, preferably operable contact-free, which is preferably actuated by a special actuation element, particularly a foil. Besides the operating sensor the device is further provided with a normmal operating unit for remote operation of the device. This normal operating unit has a plurality of activating elements for differing functions.
In the previously described known safety device there is furthermore provided a relief device for the supply line for the purposes of switching off the device as quickly as possible. This relief device is disposed at a point in the supply line distant from the device near the connection of the supply line to the supply means source. It has a relief cross section which approximately corresponds to the flow cross section of the supply line. A shut-off device for the supply line is associated with the relief device, the two devices being locked off from each other in terms of circuitry.
In the known safety device it is considerably more difficult to bypass the operation sensor than, for example, the mechanical hand lever of a deadman's switch or the like. However, a bypass is not impossible. Finally, although in the known safety device the reverse relief of the supply line, which is important for reasons of safey, is beneficial in its effects, it entails great expenditures for construction and the prevention of wear, especially if highly abrasive sandblasting means are used in a supply line.